Polymorphic Nature of Human T-Cell Leukemia Virus Type 1 Particle Cores as Revealed through Characterization of a Chronically Infected Cell Line

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 cell-to-cell transmission is dependent on the release of infectious virus particles into the virological synapse. The HTLV-1 particle structure is still poorly understood, and previous studies analyzed viruses produced by transformed lymphocytic cell lines chronically infected with HTLV-1, particularly the MT-2 cell line, which harbors truncated proviruses and expresses aberrant forms of the Gag protein. In this study, we demonstrate that the chronically infected SP cell line harbors a relatively low number of proviruses, making it a more promising experimental system for the study of the HTLV-1 particle structure. We first identified the genomic sites of integration and characterized the genetic structure of the gag region in each provirus. We also determined that despite encoding a truncated Gag protein, only the full-length Gag protein was incorporated into virus particles. Cryotransmission electron microscopy analyses of the purified virus particles revealed three classes of particles based upon capsid core morphology: complete cores, incomplete cores, and particles without distinct electron densities that would correlate with the capsid region of a core structure. Observed cores were generally polygonal, and virus particles were on average 115 nm in diameter. These data corroborate particle morphologies previously observed for MT-2 cells and provide evidence that the known poor infectivity of HTLV-1 particles may correlate with HTLV-1 particle populations containing few virus particles possessing a complete capsid core structure. IMPORTANCE Studies of retroviral particle core morphology have demonstrated a correlation between capsid core stability and the relative infectivity of the virus. In this study, we used cryo-transmission electron microscopy to demonstrate that HTLV-1 particles produced from a distinct chronically infected cell line are polymorphic in nature, with many particles lacking organized electron densities that would correlate with a complete core structure. These findings have important implications for infectious HTLV-1 spread, particularly in the context of cell-to-cell transmission, a critical step in HTLV-1 transmission and pathogenesis.KEYWORDS Gag, core morphology, cryoelectron microscopy, human retrovirus, infectivity, retrovirus assembly H uman T-cell leukemia virus type 1 (HTLV-1) was the first-described human retrovirus and is estimated to have emerged in the human population via multiple events of zoonotic transmission from monkeys to humans (1-3). Some of these

Over-representation of MEF-2 isoforms (A/C) is associated with HTLV-1-induced acute ATLL and their recruitment to 3’LTR facilitates HBZ expression from the antisense promoter via interactions with Menin and Jun D

“…Although MEF-2B mRNA levels for all cell lines were comparable to Jurkat there was a clear downregulation of MEF-2B at the protein level across all cell lines. Furthermore, we also observed similar MEF-2A expression patterns in a capillary-based electrophoresis approach, where there was increased expression of MEF-2A in all cell lines except SP cells, which is an IL-2-dependent cell line that contains only one copy of the virus in its genome (44). Similarly, M8166, a lymphoblastoid cell line that harbors a defective provirus of HTLV-1 did not demonstrate increased MEF-2A expression (45).…”

Distinct Pathway of Human T-Cell Leukemia Virus Type 1 Gag Punctum Biogenesis Provides New Insights into Enveloped Virus Assembly

“…Cell-free particle infectivity for HTLV-1 is low, and recent observations have indicated that only a portion of mature HTLV-1 particles have intact cores (57). Infectious HTLV-1 spread is more heavily dependent upon cell-to-cell transmission than that of HIV-1.…”

“…Finally, it is possible that the biogenesis of HTLV-1 Gag puncta by recruitment of Gag along the plasma membrane is a more efficient mode of particle production during cell-cell contacts. Cell-free particle infectivity for HTLV-1 is low, and recent observations have indicated that only a portion of mature HTLV-1 particles have intact cores ( 57 ). Infectious HTLV-1 spread is more heavily dependent upon cell-to-cell transmission than that of HIV-1.…”

Human T-Cell Leukemia Virus Type 1 (HTLV-1) bZIP Factor Upregulates the Expression of ICAM-1 To Facilitate HTLV-1 Infection

“…As infection of T cells by free HTLV-1 virions is extremely inefficient (10)(11)(12), T-cell infection occurs through mechanisms involving contact between an effector cell (infected with HTLV-1) and a target cell. In the context of T-cell-to-T-cell infection, initiation and stabilization of effector cell-target cell contact is mediated by intercellular adhesion molecule 1 (ICAM-1) on the surface of the effector T cell engaging with lymphocyte functioning antigen 1 (LFA-1) on the target cell (13,14).…”